How do I apply "mix" and "op" maps?
All those textures are prepared for the glTF format, but with some caveats:

The Mix texture contains Ambient Occlusion, Metalness and Roughness
The Normal map has inverted B channel (it was probably baked with flipped normals)
The &quot;OP&quot; texture is empty, so there's no need to use it at all

Since these models are made by fans in their free time, some issues are to be expected :).